Ultrasonic speakers that utilize the non-linearity of a medium (air) with respect to ultrasonic waves are known in the prior art which are able to reproduce signals of the audible frequency band that have much sharper directivity as compared with ordinary speakers.
The constitution of ultrasonic speakers is such that the carrier signal in an ultrasonic frequency band is modulated by a signal from a signal source of the audible frequency band, the resulting signal is converted into sound waves of a finite amplitude level from an ultrasonic transducer after being amplified with a power amplifier and so forth, and the sound waves are radiated through a medium (air) to reproduce a signal sound in an audible frequency range due to the nonlinear effects of the medium (air). In this case, the reproduction range of the reproduced signal in the audible frequency range is a beam-shaped range in the direction of the axis of radiation from the ultrasonic transducer (refer to, for example, Japanese Unexamined Patent Application, First Publication No. S58-119293).
However, since the sound reproduction range cannot be seen, in order to set the reproduction range as intended, it is necessary to carry out the bothersome task of confirming the setting while alternately moving the location where the sound is likely to be reproduced and the location where the speaker is placed.
In addition, as a related example of the prior art, an invention is disclosed in which an audible sound demodulation range resulting from parametric relay effects is indicated with an illumination device when ultrasonic waves that have been modulated with an audible audio signal are radiated. This invention indicates the reproduction range (audible sound demodulation range) of an ultrasonic speaker with an illumination device (spotlight) to indicate it visually. However, in this invention, since the reproduction range during installation of the ultrasonic speaker is imparted as fixed information, and there is no concept of accommodating the installation conditions or controlling the reproduction range, it does not allow volume and other information required at the time of installation to be visually confirmed (refer to, for example, Japanese Unexamined Patent Application, First Publication No. H11-27774).